1. Field of the Invention
The present invention relates to an ink composition and an image forming method.
2. Description of the Related Art
With regard to recording media for inkjet recording and ink for use in inkjet recording, various studies on techniques for obtaining high grade recorded materials in terms of, for example, color density, fixability, resolution, curling after recording, and the like have been carried out.
Pigments have been widely used as the colorant used in inkjet recording ink, in view of favorable lightfastness, water resistance, and the like. In a case in which a pigment is dispersed for use, various studies on techniques for improving the dispersed particle diameter when dispersed, stability after dispersion, uniformity of size, performance of ejection from the inkjet head, and the like have been carried out.
In connection with the above, an ink composition containing a colorant coated with a water-insoluble polymer, resin particles, and a solid moistening agent is known as an ink composition with which the bronzing phenomenon is suppressed and which exhibits favorable fixability and favorable abrasion resistance (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-273891).
Further, as an ink composition which exhibits excellent image glossiness and abrasion resistance, an ink composition containing a pigment coated with a water-insoluble polymer, and two kinds of waxes having different average particle diameters has been disclosed (see, for example, JP-A No. 2007-277290).
As an inkjet recording method with which printing unevenness is suppressed and images with high image quality and water resistance are obtained, an inkjet recording method using, as an ink composition, a recording liquid that includes resin particles and wax particles has been disclosed (see, for example, JP-A No. 2002-301857).
Moreover, an ink composition containing a pigment coated with a water-insoluble resin, resin particles, and a solid moistening agent has been disclosed (see, for example, JP-A No. 2009-221253) as an ink composition which exhibits excellent dispersion stability, effective removal of the ink by wiping, and abrasion resistance.
In recent years, with respect to inkjet recording, improvements in suitability for recording are expected in order to record at a high speed in accordance with a single pass system, which enables recording with a single operation of the inkjet head, instead of using the shuttle scanning system which has been generally used hitherto, or in order to form images on both sides of a recording medium.
In conventional inkjet methods, since the printing speed is slow, solvents in the ink dry or penetrate into the recording medium during printing, thereby ensuring the strength of the spotted ink dots. Further, due to the printing speed being slow, it was never the case that a large number of recording media were printed on at one time, or that the resulting large number of recording media were stacked in a short time.
However, when forming an image at a high speed utilizing an inkjet method, the time required for drying is short and the recording media are successively stacked, owing to which the solvents in the stacked recording media are inhibited from evaporating. As a result, the recording media are stacked while the image areas remain soft and the strength of the image areas is not ensured. This has been declared to be the cause of a phenomenon (called stacker blocking) whereby images are transferred to the reverse side of recording media printed consecutively or whereby the recording media stick to each other and damage the images. These phenomena occur more frequently as the number of stacked recording media and the area of the recording media increase and, further, the phenomena occur more frequently when recording media with a lower penetration speed are used. For example, firstly, stacker blocking does not occur in the case of printing at a low speed onto a recording medium into which ink solvents penetrate quickly, as described in JP-A No. 2000-235023.
Further, in offset printing, solid particles are generally sprayed immediately after printing and, therefore, stacker blocking hardly occurs at all.
In connection with the above, a method of printing on plain paper using an inkjet ink containing a water-soluble organic solvent having a boiling point of 200° C. or lower, a pigment, a pigment dispersant, and water has been disclosed (see, for example, JP-A No. 2007-145970).